1. Field of the Invention
The present invention relates to an X-ray CT (computed tomography) apparatus and a myocardial perfusion image generating system for generating myocardial perfusion image with continuously injecting a contrast medium.
2. Description of the Related Art
With a myocardial contrast examination using an X-ray CT apparatus, a contrast medium is injected into an object that is a patient, and contrast CT images are collected. Subsequently, moving images of coronary arteries, endocardial lumen wall, and so forth, are generated from the collected contrast CT images, and are employed for diagnosis.
Also, a blood flow dynamic state (perfusion) examination of the myocardium and a perfusion examination regarding organs such as within a brain tissue have been performed using an X-ray CT apparatus. With these perfusion examinations, attempting to generate a perfusion image by analyzing dynamic contrast CT data obtained from dynamic imaging by bolus injection for injecting a contrast medium in a short period has been conventionally studied.
Normally, such myocardial perfusion imaging is not an isolated examination, but performed as part of a contrast examination of a heart. For example, in the event of a myocardial perfusion image, scanning for heart function analysis such as coronary arteries and endocardial lumen movement is also performed as well as scanning of a myocardial perfusion image. Accordingly, it takes a long time for scanning of a myocardial perfusion image. An examination method resulting in increase of X-ray dosages for an object is hardly acceptable, and accordingly, long-period dynamic imaging using an X-ray CT apparatus has never come into practical use so far.
Furthermore, in order to perform all of the above-mentioned scans, it is necessary to increase the number of injection times and an injection amount of the contrast medium. However, an amount of the contrast medium which can be injected into an object has a limit. Accordingly, an imaging period is limited in view of not only dosage due to X-ray but also an injection amount of the contrast medium having a limit.
To such a problem, a technology to obtain blood flow information based on information including a coronary contrast CT image data and a myocardial contrast CT image data obtained by a scan for obtaining a myocardial image is devised without adding a scan for obtaining blood flow information to generate a myocardial perfusion image in more short time reducing a contrast medium injection volume to an object and exposure by X-ray.
This is a technology to extract the information related to a myocardial perfusion from a coronary contrast CT data and a myocardial contrast CT data by data processing for imaging since the information related to the myocardial perfusion serving as an index of a blood flow dynamic state on a myocardial region is included in the coronary contrast CT data and the myocardial contrast CT data acquired by a continuous contrast medium injection under a fixed condition. Specifically, during contrast medium is flowing with a constant concentration in a myocardial region and a coronary artery after static contrast medium injection into an object, a myocardial contrast CT image is obtained. Since a distribution image of contrast medium component obtained by subtracting the CT value of myocardium from the obtained myocardial contrast CT image has proportional relation to blood flow perfusion, the distribution image of contrast medium component is considered as a blood flow perfusion image indicating a relative blood flow perfusion (see, for example, Japanese Patent Application (Laid-Open) No. 2006-21022).
In addition, it is possible to convert a relative value of a myocardial blood flow image to an absolute value by calculating an unknown approximately using ECG synchronous CT image data acquired in a transitional period of concentration of contrast medium (see, for example, Japanese Patent Application (Laid-Open) No. 2006-247388). This allows generating a clinically-useful absolute value image (generally called a blood flow value image) of blood flow and a distribution image of local myocardial blood flow myocardium-wide.
As a related technology, the attempt to obtain a transforming function f(MBF) for transforming myocardial blood flow (MBF) to a ratio Cmyo/Ca between a concentration Ca of contrast medium in a left ventricle lumen of heart and a concentration Cmyo of contrast medium in blood of myocardium is performed (see, for example, George et al. Multidetector Computed Tomography Myocardial Perfusion Imaging During Adenosine Stress”, Journal of the American College of Cardiology, Vol. 48, No. 1, 2006).
In a contrast examination of a coronary artery by an X-ray CT apparatus, measuring a stenosis ratio of a thicker blood vessel portion such as the base of coronary artery and analysis in shape and aspect of a plaque in a stenosis part is becoming possible. As a result, though an imaging period subjects to constrain for imaging a myocardial perfusion image as described above, it is desired to measure a blood flow in a myocardial portion with higher accuracy without increasing an X-ray dosage and an injection amount of the contrast medium to generate a local myocardial blood flow value image with higher accuracy.
Particularly, if a contrast examination of a coronary artery and imaging of a myocardial blood flow value image with high accuracy can be performed at the same time, estimation of a stenosis in a thinner terminal coronary artery and estimation of a stenosis in a coronary artery of an object having a remarkably calcified coronary artery of which stenosis ratio is difficult to be measured, which could not be observed by an examination using an X-ray CT apparatus conventionally, become possible. Therefore, it is desired to obtain a local myocardial blood flow value image with higher accuracy without increasing dosage and contrast medium as described above.